Fixed propeller size for Yanmar sd40

[Roy Hibberd]

I have a Prout 46 Catamaran and have just lost a blade off my folding prop. As an interim measure I want to replace them with fixed props. I have 2 Yanmar 3jh3e (39hp) engines with Sd40 saildrives. The folding prop is 16ins. A Yanmar dealer has put the details in a calculator and says I need 17in props with an 11 pitch. The pitch seems very low to me, they say I won't reach full engine revs at the top end. Any thoughts from someone with a similar set up??